3.2 Summary of the Main Parameters in WCDMA

3.3 Spreading and Despreading3.4 Multipath Radio Channels and Rake Reception3.5 Power Control3.6 Softer and Soft Handovers

(1) Multiple access method◦ WCDMA is a wideband Direct-Sequence Code

Division Multiple Access (DS-CDMA) system◦ User information bits are spread over a wide

bandwidth by multiplying user data with quasi-random bits (called

chips) derived from CDMA spreading codes◦ In order to support very high bit rates (up to

2Mbps), the use of a variable spreading factor and multicode connections is supported

(2) Duplexing method◦ WCDMA supports both FDD and TDD modes of

operation◦ Frequency Division Duplex (FDD)

separate 5 MHz carrier frequencies are used for uplink and downlink, respectively

◦ Time Division Duplex (TDD) only one 5 MHz is timeshared between uplink and

downlink

(3) Basic station synchronization◦ WCDMA supports the operation of asynchronous

base stations no need for a global time reference such as a GPS deployment of indoor and micro base stations is

easier when no GPS signal needs to be received

(4) Chip rate◦ chip rate of 3.84 Mcps leads to a carrier

bandwidth (channel bandwidth) of approximately 5 MHz chip ： the length of time to transmit either a "0" or a

"1" in a binary pulse code chip rate ： number of chips per second

◦ DS-CDMA systems with a bandwidth of about 1 MHz (narrowband CDMA systems)

◦ wide carrier bandwidth of WCDMA supports high user data rates

(5) Frame length & slot length◦ frame length

10ms (1 frame length = 38400 chips)◦ slot length

15 slots /frame (1 slot length = 2560 chips)

(6) Service multiplexing◦ multiple services with different quality of service

requirements multiplexed on one connection

(7) Multirate concept◦ use a variable spreading factor and multicode to

support very high bit rates (up to 2 Mbps)◦ multicode

in multicode CDMA systems, each user can be provided with multiple spreading codes of fixed length, depending on users' rate requests

motivations for multicode CDMA increase the information rate over a given

spread bandwidth allow for the flexibility of multiple data rates

(8) Detection◦ WCDMA employs coherent detection ( 連續偵測 ) on

uplink and downlink based on the use of pilot symbols ( 導引符號 ) or common pilot ( 共用導引 )

◦ coherent detection a method of recovering the original signal that

requires an exactly same carrier frequency and phase (propagation delay causes carrier-phase offset) as those used in the transmitting end

the received signal is mixed, in some type of nonlinear device, with a signal from a local oscillator, to produce an intermediate frequency, from which the modulating signal is recovered (detected)

◦ use of coherent detection on uplink will result in an overall increase of coverage and capacity on the uplink

(9) Multiuser detection and smart antennas◦ supported by the standard◦ deployed by network operator as a system option

to increase capacity and/or coverage

Smart antennas (also known as adaptive array antennas, multiple antennas and recently MIMO)◦ antenna arrays with smart signal processing

algorithms used to identify spatial signal signature such as the direction of arrival (DOA) of the signal, and use it to calculate beamforming vectors, to track and locate the antenna beam on the mobile/target

◦ the antenna could optionally be any sensor◦ smart antenna techniques are used notably in acoustic ( 聲波的 ) signal processing, track and scan RADAR, radio astronomy ( 天文學 ) and radio telescopes ( 無線電天文望遠鏡 ), and mostly in cellular systems like W-CDMA and UMTS

WCDMA supports highly variable user data rates, in other words the concept of obtaining Bandwidth on Demand (BoD)◦ The user data rate is kept constant during each

10 ms frame◦ However, the data capacity among the users can

change from frame to frame◦ This fast radio capacity allocation will typically be

controlled by the network to achieve optimum throughput for packet data services

Handovers◦ WCDMA is designed to be deployed in conjunction

with GSM◦ handovers between GSM and WCDMA are

supported to leverage GSM coverage

Spread-spectrum transmission ◦ a technique in which the user’s original signal is

transformed into another form that occupies a larger bandwidth than the original signal would normally need

◦ the original data sequence is binary multiplied with a spreading code that typically has a much larger bandwidth than the original signal

◦ the bits in the spreading code are called chips to differentiate them from the bits in the data sequence, which are called symbols

◦ each user has its own spreading code◦ the identical code is used in both transformations

on each end of the radio channel spreading the original signal to produce a wideband

signal despreading the wideband signal back to the original

narrowband signal

◦ the ratio between the transmission bandwidth and the original bandwidth is called the processing gain also known as the spreading factor (SF) this ratio simply means how many chips are used to

spread one data symbol in the UTRAN, the spreading-factor values can be

between 4 and 512 in the TDD mode also SF=1 is allowed the lower the spreading factor, the more payload

data a signal can convey on the radio interface

Spreading and despreading operation◦ user data is assumed to be a BPSK-modulated bit

sequence of rate R◦ user data bits are assumed the values of1 or -1◦ spreading operation

the multiplication of each user data bit with a sequence of 8 code bits, called chips (the spreading factor is 8)

the resulting spread data is at a rate of 8 × R◦ despreading operation

multiply the spread user data/chip sequence, bit duration by bit duration, with the very same 8 code chips as we used during the spreading of these bits

as shown, the original user bit sequence has been recovered perfectly

the increase of the signaling rate by a factor of 8 corresponds to a widening (by a factor of 8) of the occupied spectrum of the spread user data signal

despreading restores a bandwidth proportional to R for the signal

Basic operation of the correlation receiver for CDMA ◦ the upper half of the figure shows the reception of

the desired own signal the despreading operation with a perfectly

synchronised code the correlation receiver integrates (i.e. sums) the

resulting products (data × code) for each user bit◦ the lower half of the figure shows the effect of

the despreading operation of another user with a different spreading code

the result of multiplying the interfering signal with the own code and integrating the resulting products leads to interfering signal values lingering around 0

Rake receiver◦ a radio receiver designed to counter the effects of

multipath fading uses several "sub-receivers" each delayed

slightly in order to tune in to the individual multipath components

each component is decoded independently, but at a later stage combined in order to make the most use of the different transmission characteristics of each transmission path

◦ the digital section of a CDMA receiver which permits the phone (or cell) to separate out the relevant signal from all the other signals is capable of receiving multiple signal

sources and adding them together using multiple fingers

Rake receivers are common in a wide variety of radio devices including mobile phones and wireless LAN equipment

Digitized input samples◦ received from RF (Radio Frequency) front-end

circuitry in the form of I and Q branches Code generators and correlator

◦ perform the despreading and integration to user data symbols

Channel estimator and phase rotator◦ channel estimator uses the pilot symbols for

estimating the channel state which will then be removed by the phase rotator from the received symbols

Delay equliser◦ the delay is compensated for the difference in the

arrival times of the symbols in each finger Rake combiner

◦ sums the channel compensated symbols, thereby providing multipath diversity against fading

Matched filter◦ used for determining and updating the current

multipath delay profile of the channel◦ this measured and possibly averaged multipath

delay profile is then used to assign the Rake fingers to the largest peaks

Fast power control is in particular on the uplink◦ without it, a single overpowered mobile could

block a whole cell

Power control in WCDMA◦ Open-loop power control◦ Close-loop power control

Inner-loop power control Outer-loop power control

Open loop power control in WCDMA◦ attempt to make a rough estimation of path loss

by measuring downlink beacon signal◦ problem

far too inaccurate fast fading is essentially uncorrelated between uplink

and downlink due to large frequency separation of uplink and downlink band of WCDMA FDD mode

◦ open-loop power control is used in WCDMA to provide a coarse initial power setting of MS at the beginning of a connection

Inner-loop power control in WCDMA uplink◦ BS performs frequent estimates of the received

Signal-to-Interference Ratio (SIR) and compares it to a target SIR if the measured SIR is higher than the target SIR, BS

will command MS to lower the power if SIR is too low, it will command MS to increase its

power

◦ measure–command–react cycle executed at a rate of 1500 times per second (1.5

kHz) for each MS faster than any significant change of path loss could

possibly happen faster than the fast Rayleigh fading speed for low to

moderate mobile speeds◦ inner-loop power control

prevent any power imbalance among all the uplink signals received at BS

Inner-loop power control in WCDMA downlink◦ adopt the same techniques as those used in

uplink◦ operate at a rate of 1500 times per second◦ no near–far problem due to “one cell to many

mobiles” scenario

◦ downlink closed-loop power control provide a marginal amount of additional power to MS

at the cell edge as they suffer from increased other-cell interference

enhance weak signals caused by Rayleigh fading when other error-correcting methods doesn’t work effectively

Figure 3.8 depicts closed loop transmission power control in CDMA◦ MS1 and MS2 operate within the same frequency,

separable at the BS only by their respective spreading codes

◦ it may happen that MS1 at the cell edge suffers a path loss, say 70 dB above that of MS2 which is near the BS

◦ if there were no power control mechanism for MS1 and MS2 to the same level at BS MS2 could easily overshout MS1 and thus block a

large part of the cell, giving rise to the near–far problem of CDMA

Figure 3.9 shows how uplink closed loop power control works on a fading channel at low speed

Outer-loop power control in WCDMA◦ adjusts the target SIR setpoint in BS according to

the individual radio link quality requirement, usually defined as bit error rate (BER) or block error rate (BLER)

◦ the required SIR or BLER depends on the mobile speed, multipath profile, and data rate

◦ should the transmission quality is decreasing, the RNC will command Node B to increase the target SIR

◦ outer-loop power control is implemented in RNC because there might be soft handover combining

Why should there be a need for changing the target SIR setpoint? ◦ the required SIR for, say, BLER = 1% depends on

mobile speed and multipath profile◦ if one were to set the target SIR setpoint for high

mobile speeds, one would waste much capacity for those connections at low speeds

◦ the best strategy is to let the target SIR setpoint float around the minimum value that just fulfils the required target quality

The target SIR setpoint will change over time as the speed and propagation environment changes (Figure 3.10)

Outer loop control is typically implemented by◦ having BS tag each uplink user data frame with a

frame reliability indicator, such as a CRC (Cyclic Redundancy Check) result obtained during decoding of that particular user data frame

◦ should the frame quality indicator shows the transmission quality is decreasing RNC will command BS to increase target SIR setpoint

Softer handover (Figure 3.11)◦ MS is in the overlapping cell coverage area of two

adjacent sectors of a BS◦ communications between MS and BS take place

concurrently via two air interface channels, one for each sector separately

◦ use of two separate codes in the downlink direction, so MS can distinguish the signals

◦ the signals are received in the MS by means of Rake processing, and the fingers need to generate the respective code for each sector for the appropriate despreading operation due to multipath propagation, it is necessary to use

multiple correlation receivers in order to recover the energy from all paths and/or antennas

such a collection of correlation receivers, termed ‘fingers’, is what comprises the CDMA Rake receiver

◦ only one power control loop per connection is active

◦ softer handover typically occurs in about 5~15% of connections

In the uplink direction ◦ the code channel of MS is received in each sector◦ use maximal ratio combining (MRC) Rake

processing

Maximal Ratio Combiner (MRC)◦ the combiner that achieves the best performance

is one in which each output is multiplied by the corresponding complex-valued (conjugate) channel gain

◦ the effect of this multiplication is to compensate for the phase shift in the channel and to weight the signal by a factor that is proportional to signal strength

Soft handover (Figure 3.12)◦ MS is in the overlapping cell coverage area of two

sectors belonging to different BSs◦ communications between MS and BS take place

concurrently via two air interface channels from each BS separately

◦ both channels (signals) are received at the MS by maximal ratio combining Rake processing

◦ two power control loops per connection are active, one for each BS

◦ soft handover occurs in about 20~40% of connections

In the uplink direction◦ the code channel of the MS is received from both

BSs, but the received data is then routed to RNC for combining

◦ the same frame reliability indicator is used to select the better frame between the two possible candidates within RNC

◦ this selection takes place every 10~80 ms

Soft and softer handover can take place in combination with each other

Other handover types of WCDMA◦ Inter-frequency hard handovers

e.g., to hand a mobile over from one WCDMA frequency carrier to another

one application for this is high capacity BSs with several carriers

◦ Inter-system hard handover takes place between WCDMA FDD system and

another system, such as WCDMA TDD or GSM